Golf ball

ABSTRACT

The invention provides a golf ball having a solid core, a cover of at least one layer, and a plurality of dimples on a surface of an outermost layer of the cover. The dimples number at least 250 but not more than 500, and have a volume ratio (VR) of from 1.00 to 1.40%. The ball has a deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of from 4.0 to 6.0 mm, and has an initial velocity, as measured using an initial velocity measuring apparatus of the same type as a USGA drum rotation-type initial velocity instrument, of from 70.0 to 76.0 m/s. This golf ball is able to substantially reduce the distance traveled by the ball when struck at a high head speed, while at the same time holding down the decrease in distance when struck at a low head speed.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball which is composed of a solid core and a cover of at least one layer, and which has a plurality of dimples on a surface of the outermost layer of the cover. More specifically, the invention relates to a golf ball which substantially reduces the distance traveled by the ball when struck at a high head speed (head speed is sometimes abbreviated below as “HS”) while at the same time minimizing the degree of reduction in distance when struck at a low head speed.

With recent advances in golfing equipment such as balls and clubs, golf balls have come to travel increasing distances. For this reason, to keep play fair, strict rules have been adopted which establish, in the case of a golf club, for example, the size of the head and the length of the shaft. Similarly, limitations have been placed on certain characteristics of the golf ball, such as its size, weight and initial velocity, so as to restrict excessive ball travel of the sort that would result in a loss of fair play.

The trend toward regulation is accelerating for golf balls as well, and there is a possibility that the upper limit in the distance traveled by a golf ball under the conditions of use by a professional golfer, i.e., under high HS conditions, will be further restricted. Generally, if the distance a ball travels when played under high HS conditions is reduced, the distance traveled by the ball when played under the conditions of use by an ordinary amateur player, i.e., under low HS conditions, also ends up decreasing to a similar degree. Accordingly, there exists a desire to satisfy the above restriction while at the same time minimizing the decrease in distance by the golf ball when used by an ordinary amateur player.

The distance traveled by a golf ball is generally held down by limiting the initial velocity. However, in such cases, the distance often decreases in about the same ratio both at high head speeds and low head speeds. As a result, such balls have significant drawbacks for low HS players.

As another approach, a variety of golf balls have been disclosed which, by optimizing the dimples on the surface of the ball, lower the flight trajectory and hold down the decrease in distance.

For example, JP-A 05-103846 describes a golf ball in which the dimple diameter, dimple depth and number of dimples have been optimized. JP-A 10-043342 and JP-A 10-043343 disclose golf balls in which the amount of deformation by a ball when compressed under a load of 100 kgf has been optimized, along with which the dimple diameter divided by the dimple depth has been set to from 10 to 15 or the dimple space volume as a proportion of the total volume of a hypothetical sphere were the surface of the ball to have no dimples thereon has been set to from 0.7 to 1.1%. JP-A 2000-107338 discloses a practice golf ball having an optimized ball weight and diameter.

It is therefore an object of the present invention to provide a golf ball which can achieve a superior distance in a low HS range while holding down the distance traveled in a high HS range.

SUMMARY OF THE INVENTION

The inventors have conducted extensive investigations in order to achieve the above object. As a result, they have found that, in a golf ball which is composed of a solid core and a cover of at least one layer, and which has a plurality of dimples on a surface of the outermost layer of the cover, by specifying, for the dimples formed on the surface of the outermost cover layer, the number of dimples and the dimple volume ratio (VR), and by optimizing the deflection and initial velocity of the ball as a whole, the distance traveled by the ball when struck at a high head speed can be substantially reduced while at the same time holding down the decrease in distance when the ball is struck at a low head speed.

That is, unlike conventional methods which lower the ball initial velocity or core initial velocity, the golf ball of the present invention is able, by combining low-trajectory dimples with the internal structure of the ball, to substantially reduce the distance traveled by the ball when struck at a high head speed while at the same time holding down as much as possible, relative to the reduction in distance at high head speed, the decrease in the distance traveled by the ball when struck at a low head speed. As used herein, “distance” refers to the total distance traveled by a golf ball, including both the carry and the run.

Accordingly, the invention provides the following golf balls.

[1] A golf ball comprising a solid core, a cover of at least one layer, and a plurality of dimples on a surface of an outermost layer of the cover, wherein the dimples number at least 250 but not more than 500 and have a volume ratio (VR) of from 1.00 to 1.40%, and the ball has a deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of from 4.0 to 6.0 mm and an initial velocity, as measured using an initial velocity measuring apparatus of the same type as a USGA drum rotation-type initial velocity instrument, of from 70.0 to 76.0 m/s. [2] The golf ball of [1], wherein the ball, under the conditions of a head speed of 54 m/s, a ball initial velocity of 78.0±0.5 m/s, a launch angle of 9.7±0.5° and an initial backspin rate of 2,700±100 rpm, has a total distance of not more than 290 yards. [3] The golf ball of [1], wherein the ball has a ratio of total distance traveled when struck at a head speed of 54 m/s to total distance traveled when struck at a head speed of 35 m/s (HS54/HS35) of from 1.30 to 1.50. [4] The golf ball of [1], wherein the dimple volume ratio and the deflection satisfy the relationship expressed by the formula:

4×VR+deflection=8.0 to 10.0.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing an example of the internal structure of the golf ball according to the present invention.

FIG. 2 is a schematic view illustrating a dimple used in the present invention.

FIG. 3 is a top view showing a dimple pattern (I) used on golf balls in examples of the invention and comparative examples.

FIG. 4 is a top view showing a dimple pattern (II) used on a golf ball in an example of the invention.

FIG. 5 is a top view showing a dimple pattern (III) used on a golf ball in a comparative example.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention is a multi solid golf ball having a solid core (referred to below as simply the “core”) and a cover of at least one layer. A plurality of dimples are formed on a surface of an outermost layer of the cover. By having a deflection and an initial velocity for the ball as a whole within specific ranges and combining therewith dimples which satisfy the subsequently described specific parameters, the distance traveled by the ball when struck at a high head speed can be substantially reduced while holding down the decrease in the distance traveled by the ball when struck at a low head speed. As used in the present invention, “high HS range” refers to a range of about 50 to 60 m/s and “low HS range” refers to a range of 30 to 40 m/s.

The internal structure of the golf ball G of the present invention need only have a core and a cover of at least one layer, and may be suitably set without particular limitation within a range that does not depart from the objects of the invention. For example, when the ball is a three-piece solid golf ball having a two-layer cover composed of an inner layer and an outer layer, as shown in FIG. 1, the ball has a three-layer construction with at least a core 1, an inner cover layer 2 which encases the core 1, and an outer cover layer 3 which encases the inner cover layer 2. When the ball has a multi-layer construction with a cover of two or more layers, the cover layers are sometimes referred to collectively herein as the “cover.” That is, in the case of the three-piece solid golf ball shown in FIG. 1, the inner cover layer 2 and the outer cover layer 3 are sometimes referred to collectively as the “cover.” A plurality of dimples D are generally formed on the surface of the outer cover layer 3, and these dimples D satisfy the subsequently described parameters of the invention. It should be noted that, although FIG. 1 shows a three-layer construction arrived at by forming a core, an inner cover layer 2 and an outer cover layer 3, as mentioned above, this arrangement may be suitably modified within a range that does not depart from the objects of the invention. For example, if necessary, the cover may be formed of one layer or of three or more layers. Or the core 1 may be formed of a plurality of layers.

The core in the invention may be formed using a rubber composition containing, for example, a base rubber and also such ingredients as a co-crosslinking agent, an organic peroxide, an inert filler, sulfur and an organosulfur compound. The base rubber of the rubber composition is preferably one composed primarily of a known polybutadiene.

In the present invention, an organosulfur compound may be optionally included in the base rubber in order to increase the rebound of the core. When an organosulfur compound is included, the amount of organosulfur compound per 100 parts by weight of the base rubber may be set to preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and even more preferably at least 0.2 part by weight. The upper limit in the amount included may be set to preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight by weight, and even more preferably not more than 2 parts by weight. If the amount of organosulfur compound included is too small, a sufficient core rebound-increasing effect may not be obtained. On the other hand, if too much organosulfur compound is included, the core may become too soft, resulting in a poor feel when the ball is played and a poor durability to cracking on repeated impact.

The diameter of the core, although not subject to any particular limitation, may be set to from 30 to 42 mm. In this case, the lower limit value is preferably at least 32 mm, more preferably at least 34 mm, and even more preferably at least 35 mm. The upper limit value may be set to preferably not more than 41 mm, more preferably not more than 40 mm, even more preferably not more than 39 mm, and most preferably not more than 38 mm.

The core deflection, i.e., the amount of deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), although not subject to any particular limitation, may be set within a range of from 3.0 to 9.0 mm. In this case, the lower limit value is preferably at least 3.5 mm, more preferably at least 4.0 mm, and even more preferably at least 4.5 mm. The upper limit value may be set to preferably not more than 8.0 mm, and more preferably not more than 7.0 mm. If the core is too much harder than the above range (small deflection), a sufficient distance-reducing effect may not be achievable on shots taken at a high head speed. On the other hand, if the core is too much softer than the above range (large deflection), the feel of the ball may become too soft and the durability to cracking on repeated impact may worsen.

The specific gravity of the core, while not subject to any particular limitation, may be set within a range of from 0.9 to 1.4. In such a case, the lower limit value is preferably at least 1.0, and more preferably at least 1.1. The upper limit value may be set to preferably not more than 1.3, and more preferably not more than 1.2.

In the present invention, by using the above material to form the solid core 1, the rebound can be increased, thus enabling a golf ball capable of achieving a stable trajectory to be provided.

The golf ball G of the present invention has a cover of at least one layer formed over the solid core 1. The number, material hardness (Shore D) and thickness of the cover layers formed in this invention are not subject to any particular limitation, and may be set as appropriate within ranges that do not depart from the objects of the invention. For example, when the three-piece solid golf ball shown in FIG. 1 is manufactured by forming over the core 1 a two-layer cover composed of an inner cover layer 2 and an outer cover layer 3, these parameters may be set as indicated below. Here, “material hardness (Shore D)” refers to the hardness of a sheet of the cover material that has been formed under applied pressure to a thickness of about 2 mm, as measured using a type D durometer in general accordance with ASTM D2240.

First, the material hardness (Shore D) of the inner cover layer, although not subject to any particular limitation, may be set to at least 30, preferably at least 35, more preferably at least 40, and most preferably at least 45. It is recommended that the upper limit be not more than 66, preferably not more than 63, and more preferably not more than 60. When the material hardness (Shore D) of the inner cover layer is too high, the ball may have a poor feel on impact.

The thickness of the inner cover layer, although not subject to any particular limitation, may be set to at least 0.5 mm, preferably at least 0.7 mm, more preferably at least 1.0 mm, and still more preferably at least 1.3 mm. It is recommended that the upper limit be not more than 3.0 mm, preferably not more than 2.5 mm, even more preferably not more than 2.3 mm, and most preferably not more than 2.2 mm. When the inner cover layer is too thin, the durability may worsen; when it is too thick, the ball may have a poor feel on impact.

The material hardness (Shore D) of the outer cover layer, although not subject to any particular limitation, may be set to at least 35, preferably at least 40, more preferably at least 43, and still more preferably at least 46. It is recommended that the upper limit be not more than 65, preferably not more than 63, more preferably not more than 61, and most preferably not more than 60. If the material hardness (Shore D) of the outer cover layer is too low, the feel on impact may be too soft. On the other hand, if it is too high, the durability or the feel on impact may worsen.

The thickness of the outer cover layer, although not subject to any particular limitation, may be set to at least 0.5 mm, preferably at least 0.7 mm, and more preferably at least 0.8 mm. It is recommended that the upper limit be not more than 3.0 mm, preferably not more than 2.5 mm, more preferably not more than 2.0 mm, and even preferably not more than 1.6 mm. If the thickness of the outer cover layer falls outside the above range, this may lead to a worsening in the feel of the ball on impact or in the durability.

In the present invention, the cover may be formed of a known material, examples of which include, but are not limited to, thermoplastic resins such as ionomeric resins, and various types of thermoplastic elastomers. Examples of thermoplastic elastomers include polyester-based thermoplastic elastomers, polyamide-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers.

In the present invention, such cover materials are not subject to any particular limitation, although preferred use may be made of a cover material composed primarily of a material selected from the group consisting of the polyurethane materials (I), polyurethane materials (II) and ionomeric resin materials shown below. These materials, including molding methods for the same, are described in order below.

Polyurethane Material (I)

This material (I) is composed primarily of components A and B below:

(A) a thermoplastic polyurethane material, (B) an isocyanate mixture obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate.

Golf balls in which the cover has been formed of this material (I) can be endowed with an excellent feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact.

Next, each of the above components is described.

The thermoplastic polyurethane material (A) has a structure which includes soft segments made of a polymeric polyol (polymeric glycol), and hard segments made of a chain extender and a diisocyanate. Here, the polymeric polyol serving as a starting material is not subject to any particular limitation, and may be any that has hitherto been used in the art relating to thermoplastic polyurethane materials, such as polyester polyols and polyether polyols. Polyether polyols are preferable to polyester polyols because they enable the synthesis of thermoplastic polyurethane materials having a high rebound resilience and excellent low-temperature properties. Illustrative examples of polyether polyols include polytetramethylene glycol and polypropylene glycol. From the standpoint of rebound resilience and low-temperature properties, polytetramethylene glycol is especially preferred. The polymeric polyol has an average molecular weight of preferably from 1,000 to 5,000. A molecular weight of from 2,000 to 4,000 is especially preferred for synthesizing thermoplastic polyurethane materials having a high rebound resilience.

The chain extender employed is preferably one that has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. These chain extenders have an average molecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one that has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, for stable reactivity with the subsequently described isocyanate mixture (B), it is most preferable to use the following aromatic diisocyanate: 4,4′-diphenylmethane diisocyanate.

A commercial product may be advantageously used as the thermoplastic polyurethane material composed of the above-described material. Illustrative examples include those available under the trade names Pandex T-8290, Pandex T-8295 and Pandex T8260 (DIC Bayer Polymer, Ltd.), and those available under the trade names Resamine 2593 and Resamine 2597 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.).

The isocyanate mixture (B) is obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate. Here, the isocyanate compound (B-1) is preferably an isocyanate compound that has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. From the standpoint of reactivity and work safety, the use of 4,4′-diphenylmethane diisocyanate is most preferred.

The thermoplastic resin (B-2) is preferably a resin having a low water absorption and excellent compatibility with thermoplastic polyurethane materials. Illustrative examples of such resins include polystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonate resins, and polyester elastomers (e.g., polyether-ester block copolymers, polyester-ester block copolymers). From the standpoint of the rebound resilience and strength, the use of a polyester elastomer, particularly a polyether-ester block copolymer, is especially preferred.

In the isocyanate mixture (B), it is desirable for the relative proportions of the thermoplastic resin (B-2) and the isocyanate compound (B-1), expressed as the weight ratio (B-2):(B-1), to be from 100:5 to 100:100, and especially from 100:10 to 100:40. If the amount of the isocyanate compound (B-1) relative to the thermoplastic resin (B-2) is too small, a greater amount of the isocyanate mixture (B) will have to be added to achieve an amount of addition sufficient for the crosslinking reaction with the thermoplastic polyurethane material (A). As a result, the thermoplastic resin (B-2) will exert a large influence, rendering the physical properties of the material inadequate. On the other hand, if the amount of the isocyanate compound (B-1) relative to the thermoplastic resin (B-2) is too large, the isocyanate compound (B-1) may cause slippage to occur during mixing, making preparation of the isocyanate mixture (B) difficult.

The isocyanate mixture (B) may be obtained by, for example, adding the isocyanate compound (B-1) to the thermoplastic resin (B-2) and thoroughly working together these components at a temperature of from 130 to 250° C. using mixing rolls or a Banbury mixer, then either pelletizing or cooling and subsequently grinding. A commercial product such as that available under the trade name Crossnate EM30 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.) may be suitably used as the isocyanate mixture (B).

The above material (I) is composed primarily of the thermoplastic polyurethane material (A) and isocyanate mixture (B) described above. In this material (I), the isocyanate mixture (B) is included in an amount, per 100 parts by weight of the thermoplastic polyurethane material (A), of at least 1 part by weight, preferably at least 5 parts by weight, and more preferably at least 10 parts by weight, but not more than 100 parts by weight, preferably not more than 50 parts by weight, and more preferably not more than 30 parts by weight. If too little isocyanate mixture (B) is included relative to the thermoplastic polyurethane material (A), a sufficient crosslinking effect will not be achieved. On the other hand, if too much is included, this may lead to discoloration of the molded material by unreacted isocyanate, which is undesirable.

In addition to above components (A) and (B), another component (C), although not essential, may also be included in the material (I). This other component is exemplified by thermoplastic polymeric materials other than thermoplastic polyurethane materials; illustrative examples include polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, polyethylene, and nylon resins. When component (C) is included, the amount is not subject to any particular limitation and may be suitably selected as appropriate for such purposes as adjusting the hardness, improving the resilience, improving the flow properties, and improving the adhesion of the cover material. The amount of component (C) included per 100 parts by weight of component (A) is set to preferably at least 10 parts by weight, and the upper limit is set to not more than 100 parts by weight, preferably not more than 75 parts by weight, and more preferably not more than 50 parts by weight. If necessary, various additives such as pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may also be suitably included in the above material (I).

Formation of the cover using the above material (I) may be carried out by a known molding method. For example, the cover may be molded by adding the isocyanate mixture (B) to the thermoplastic polyurethane material (A) and dry mixing, feeding the resulting mixture to an injection molding machine, and injecting the molten resin blend over the core. In such a case, the molding temperature differs with the type of thermoplastic polyurethane material (A), although molding is generally carried out within a temperature range of 150 to 250° C.

Reactions and crosslinking which take place in the golf ball cover obtained as described above are believed to involve the reaction of isocyanate groups with hydroxyl groups remaining in the thermoplastic polyurethane material to form urethane bonds, or the creation of an allophanate or biuret crosslinked form via a reaction involving the addition of isocyanate groups to urethane groups in the thermoplastic polyurethane material. Although the crosslinking reaction has not yet proceeded to a sufficient degree immediately after injection molding of the material (I), the crosslinking reaction can be made to proceed further by carrying out an annealing step after molding, in this way conferring the golf ball cover with useful characteristics. “Annealing,” as used herein, refers to heat aging the cover at a constant temperature for a fixed length of time, or aging the cover for a fixed period at room temperature.

Polyurethane Material (II)

This material (II) is a single resin blend in which the primary components are (D) a thermoplastic polyurethane and (E) a polyisocyanate compound. By forming a cover composed primarily of such a polyurethane material (II), it is possible to achieve an excellent feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact without a loss of resilience.

As used herein, reference to a “single” resin blend means that the resin blend is not fed as a plurality of types of pellets, but rather is supplied to, for example, an injection molding machine as one type of pellet prepared by incorporating a plurality of ingredients into individual pellets.

To fully and effectively achieve the objects of the invention, a necessary and sufficient amount of unreacted isocyanate groups should be present within the cover resin material. Specifically, it is recommended that the combined weight of above components (D) and (E) account for at least 60%, and preferably at least 70%, of the total weight of the cover. Components (D) and (E) are described in detail below.

The above thermoplastic polyurethane (D) is described. The thermoplastic polyurethane structure includes soft segments made of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments made of a chain extender and a polyisocyanate compound. Here, the long-chain polyol used as a starting material is not subject to any particular limitation, and may be any that has hitherto been used in the art relating to thermoplastic polyurethanes. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof. Of the long-chain polyols mentioned here, polyether polyols are preferred because they enable the synthesis of thermoplastic polyurethanes having a high rebound resilience and excellent low-temperature properties.

Illustrative examples of the above polyether polyol include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol) obtained by the ring-opening polymerization of cyclic ethers. These polyether polyols may be used singly or as a combination of two or more thereof. In the present invention, poly(tetramethylene glycol) and poly(methyltetramethylene glycol) are preferred.

It is preferable for these long-chain polyols to have a number-average molecular weight in a range of 1,500 to 5,000. By using a long-chain polyol having a number-average molecular weight within this range, golf balls made with a thermoplastic polyurethane composition having excellent properties such as resilience and manufacturability can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in a range of 1,700 to 4,000, and even more preferably in a range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chain polyol” refers to the number-average molecular weight calculated based on the hydroxyl number measured in accordance with JIS K-1557.

Any chain extender employed in the prior art relating to thermoplastic polyurethane materials may be advantageously used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 400 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. In the present invention, an aliphatic diol having 2 to 12 carbons is preferred, and 1,4-butylene glycol is more preferred.

Any polyisocyanate compound hitherto employed in the art relating to thermoplastic polyurethane materials may be advantageously used without particular limitation as the polyisocyanate compound. For example, use may be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving as above component D to be a thermoplastic polyurethane synthesized using a polyether polyol as the long-chain polyol, using an aliphatic diol as the chain extender, and using an aromatic diisocyanate as the polyisocyanate compound. It is desirable, though not essential, for the polyether polyol to be polytetramethylene glycol having a number-average molecular weight of at least 1,900, for the chain extender to be 1,4-butylene glycol, and for the polyisocyanate compound to be 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in the above polyurethane-forming reaction can be adjusted within a desirable range so as to make it possible to obtain a golf ball which is composed of a thermoplastic polyurethane composition and has various improved properties, such as rebound, spin performance, scuff resistance and manufacturability. Specifically, in preparing a thermoplastic polyurethane by reacting the above long-chain polyol, polyisocyanate compound and chain extender, it is desirable to use the respective components in proportions such that the amount of isocyanate groups on the polyisocyanate compound per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing component (D). Production may be carried out by either a prepolymer process or a one-shot process in which the long-chain polyol, chain extender and polyisocyanate compound are used and a known urethane-forming reaction is effected. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

A commercial product may be used as component (D). Illustrative examples include products available under the trade names Pandex T8295, Pandex T8290 and Pandex T8260 (DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component E, it is essential that, in at least some portion thereof within a single resin blend, all the isocyanate groups on the molecule remain in an unreacted state. That is, polyisocyanate compound in which all the isocyanate groups on the molecule are in a completely free state should be present within a single resin blend, and such a polyisocyanate compound may be present together with a polyisocyanate compound in which a portion of the isocyanate groups on the molecule are in a free state.

Various isocyanates may be used without particular limitation as the polyisocyanate compound. Specific examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Of the above group of isocyanates, using 4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate is preferred for achieving a good balance between the influence on moldability by, for example, the rise in viscosity associated with reaction with the thermoplastic polyurethane serving as component D, and the properties of the resulting golf ball cover material.

In material (II), although not an essential ingredient, a thermoplastic elastomer other than the above thermoplastic polyurethane may be included as component F in addition to above components D and E. Including this component F in the above resin blend enables the flow properties of the resin blend to be further improved and enables various properties required of golf ball cover materials, such as resilience and scuff resistance, to be enhanced.

This component F, which is a thermoplastic elastomer other than the above thermoplastic polyurethane, is exemplified by one or more thermoplastic elastomer selected from among polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins. The use of polyester elastomers, polyamide elastomers and polyacetals is especially preferred because the resilience and scuff resistance are enhanced, owing to reactions with isocyanate groups, while at the same time a good manufacturability is retained.

The relative proportions of above components D, E and F are not subject to any particular limitation. However, to fully achieve the advantageous effects of the invention, it is preferable for the weight ratio among the respective components to be (D):(E):(F)=100:2 to 50:0 to 50, and more preferably (D):(E):(F)=100:2 to 30:8 to 50.

In this invention, a single resin blend for forming the cover is prepared by mixing together component D, component E, and also optional component F. At this time, it is essential to select the mixing conditions such that, of the polyisocyanate compound, at least some polyisocyanate compound is present in which all the isocyanate groups on the molecule remain in an unreacted state. For example, treatment such as mixture in an inert gas (e.g., nitrogen) or in a vacuum state must be furnished. The resin blend is then injection-molded over a core which has been placed in a mold. To smoothly and easily handle the resin blend, it is preferable for the blend to be formed into pellets having a length of 1 to 10 mm and a diameter of 0.5 to 5 mm. Sufficient isocyanate groups in an unreacted state remain in these resin pellets; the unreacted isocyanate groups react with component D or component F to form a crosslinked material while the resin blend is being injection-molded about the core, or due to post-treatment such as annealing thereafter.

In addition, various optional additives may also be included in this cover-forming resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers, and parting agents may be suitably included.

The melt mass flow rate (MFR) of this resin blend at 210° C. is not subject to any particular limitation. However, to increase the flow properties and manufacturability, the MFR is preferably at least 5 g/10 min, and more preferably at least 6 g/10 min. If the melt mass flow rate of the resin blend is too low, the flow properties will decrease, which may cause eccentricity during injection molding and may also lower the degree of freedom in the thickness of the cover that can be molded. The melt mass flow rate is a measured value obtained in accordance with JIS K-7210 (1999 edition).

The method of molding the cover may involve feeding the above resin blend to an injection-molding machine and injecting the molten resin blend over the core. Although the molding temperature in this case will vary depending on the type of thermoplastic polyurethane, the molding temperature is generally from 150 to 250° C.

When injection molding is carried out, it is desirable though not essential to carry out molding in a low-humidity environment such as by purging with an inert gas (e.g., nitrogen) or a low-moisture gas (e.g., low dew-point dry air), or vacuum treating, some or all places on the resin paths from the resin feed area to the mold interior. Illustrative, non-limiting, examples of the medium used for transporting the resin include low-moisture gases such as low dew-point dry air or nitrogen. By carrying out molding in such a low-humidity environment, reaction by the isocyanate groups is kept from proceeding before the resin has been charged into the mold interior. As a result, polyisocyanate in which the isocyanate groups are present in an unreacted state is included to some degree in the molded resin material, thus making it possible to reduce variable factors such as an unnecessary rise in viscosity and enabling the real crosslinking efficiency to be enhanced.

Techniques that may be used to confirm the presence of polyisocyanate compound in an unreacted state within the resin blend prior to injection molding about the core include those which involve extraction with a suitable solvent that selectively dissolves out only the polyisocyanate compound. An example of a simple and convenient method is one in which confirmation is carried out by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) measurement in an inert atmosphere. For example, when the above-described single resin blend (material (II)) is heated in a nitrogen atmosphere at a temperature ramp-up rate of 10° C./min, a gradual drop in the weight of diphenylmethane diisocyanate can be observed from about 150° C. On the other hand, in a resin sample in which the reaction between the thermoplastic polyurethane material and the isocyanate mixture has been carried out to completion, a weight drop is not observed from about 150° C., but a weight drop can be observed from about 230 to 240° C.

After the above material (II) has been injection-molded to form a cover, the properties as a golf ball cover can be additionally improved by carrying out annealing so as to induce the crosslinking reaction to proceed further. “Annealing,” as used herein, refers to aging the cover in a fixed environment for a fixed length of time.

Ionomeric Resin Material

In the present invention, “ionomeric resin material” refers to a resin composition which is composed primarily of a metal salt of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymer and/or a metal salt of an olefin-unsaturated carboxylic acid random copolymer.

The olefin generally has a number of carbons that is at least 2, but not more than 8, and preferably not more than 6. Illustrative examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Illustrative examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester may be, for example, a lower alkyl ester of an unsaturated carboxylic acid. Illustrative examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The use of butyl acrylate (n-butyl acrylate, isobutyl acrylate) is especially preferred.

The random copolymer may be obtained by the random copolymerization of the above ingredients in accordance with a known method. Here, the unsaturated carboxylic acid content (acid content) within the random copolymer, although not subject to any particular limitation, may be set to generally at least 2 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %. It is recommended that the upper limit in the unsaturated carboxylic acid content (acid content), although not subject to any particular limitation, be generally not more than 25 wt %, preferably not more than 20 wt %, and more preferably not more than 15 wt %. At a low acid content, the rebound may decrease, whereas at a high acid content, the processability of the material may decrease.

Some or all of the acid groups in the random copolymer are neutralized with metal ions. Although the degree of neutralization in this case is not subject to any particular limitation, it is recommended that at least 20 mol %, preferably at least 30 mol %, more preferably at least 40 mol %, and even more preferably at least 70 mol %, of the acid groups be neutralized. The upper limit in the degree of neutralization, although not subject to any particular limitation, may be set to 100 mol % or less, preferably 95 mol % or less, and more preferably 90 mol % or less. At a degree of neutralization below 20%, the rebound may decrease. Here, the metal ions which neutralize the acid groups are exemplified by Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. In the present invention, of these, Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ are especially preferred.

No particular limitation is imposed on the content of the above metal salt of a random copolymer (ionomeric resin), although the metal salt is preferably included in an amount of from 100 to 50 wt % based on the overall resin composition. In this case, the lower limit is more preferably at least 60 wt %, even more preferably at least 70 wt %, and most preferably at least 80 wt %. The upper limit is more preferably 95 wt % or less, and even more preferably 90 wt % or less. In this invention, a known material may be used as the ionomeric resin material. Specific examples include those available from E.I. DuPont de Nemours & Co. under the trade names HPF 1000 and HPF 2000, and the resin compositions mentioned in U.S. patent application Ser. No. 12/340,790 (or U.S. patent application Ser. No. 12/706,175). These may be used singly or as mixtures of two or more thereof.

To further improve the feel of the ball on impact, various non-ionomeric thermoplastic elastomers may also be included. Such non-ionomeric thermoplastic elastomers are exemplified by olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, ester-based thermoplastic elastomers and urethane-based thermoplastic elastomers. In the present invention, the use of an olefin-based thermoplastic elastomer is especially preferred. The olefin-based thermoplastic elastomer is a thermoplastic block copolymer having a crystalline polyolefin block and a polyethylene/butylene random copolymer. This olefin-based thermoplastic elastomer is exemplified by thermoplastic block copolymers composed of a crystalline polyethylene block (E) as a hard segment and a block of a relatively random copolymer of ethylene and butylene (EB) as a soft segment. Preferred use may be made of block copolymers having a molecular structure with a hard segment at one or both ends, such as block copolymers having an E-EB or E-EB-E structure.

These may be obtained by, for example, hydrogenating a polybutadiene. Here, the polybutadiene used in hydrogenation is preferably one in which bonding within the butadiene structure is characterized by a 1,4-bond content in the overall butadiene structure of from 95 to 100 wt %, and in which from 50 to 100 wt %, and preferably from 80 to 100 wt %, of the 1,4-bonds are present as block-like regions.

The above-mentioned E-EB-E type thermoplastic block copolymer is preferably one obtained by hydrogenating a polybutadiene having at both ends of the molecular chain 1,4-polymerization products which are rich in 1,4-bonds and having an intermediate region where 1,4-bonds and 1,2-bonds are intermingled. The degree of hydrogenation (conversion of double bonds on the polybutadiene to saturated bonds) in the polybutadiene hydrogenate is preferably from 60 to 100%, and more preferably from 90 to 100%. Too low a degree of hydrogenation may give rise to undesirable effects such as gelation in the blending step with other components such as an ionomeric resin and, when the golf ball has been formed, may lead to a poor durability to impact.

In the block copolymer having an E-EB or E-EB-E molecular structure with a hard segment at one or both ends that may be advantageously used as the thermoplastic block copolymer, the content of the hard segments is preferably from 10 to 50 wt %. If the hard segment content is too high, the cover may lack sufficient softness, making it difficult to effectively achieve the objects of the invention. On the other hand, if the hard segment content is too low, the blend may have a poor moldability.

The thermoplastic block copolymer has a melt mass flow index, at a test temperature of 230° C. and a test load of 21.2 N, of preferably from 0.01 to 15 g/10 min, and more preferably from 0.03 to 10 g/10 min. Outside of this range, problems such as weld lines, sink marks and short shots may arise during injection molding. Moreover, it is preferable for the thermoplastic block copolymer to have a surface hardness of from 10 to 50. If the surface hardness is too low, the golf ball may have a decreased durability to repeated impact. On the other hand, if the surface hardness is too high, a blend of the thermoplastic block copolymer with an ionomeric resin may have a decreased rebound. The thermoplastic block copolymer has a number-average molecular weight of preferably from 30,000 to 800,000.

A commercial product may be used as the olefin-based thermoplastic elastomer. Illustrative examples include those available under the trade names Dynaron 6100P, Dynaron 6200P and Dynaron 6201B (JSR Corporation). Of these, Dynaron 6100P, which is a block polymer having crystalline olefin blocks at both ends, is especially preferred for use in the present invention. These olefinic thermoplastic elastomers may be used singly or as mixtures of two or more thereof.

Various additives may also be optionally included in the above resin composition. Examples of additives which may be suitably included are pigments, dispersants, antioxidants, ultraviolet absorbers and optical stabilizers.

The cover material used in the invention may be a known cover material. Although not subject to any particular limitation, preferred use may be made of the above-described polyurethane material (I), polyurethane material (II) or ionomeric resin material.

In the golf ball of the invention, by specifying the deflection and initial velocity of the ball as a whole and also forming dimples which satisfy the subsequently described specific parameters and are able to achieve a relatively low trajectory, it is possible to greatly reduce the distance traveled by the golf ball on shots taken at a high head speed while also holding down the decrease in distance traveled by the ball on shots taken at a low head speed. The parameters for the dimples formed on the inventive golf ball are described in detail below.

In the present invention, dimples having the following parameters (1) and (2) are formed on the surface of the cover formed of the above-described material. In cases where the surface of the ball is subjected to finishing treatment (e.g., painting and stamping) after the cover has been formed, parameters (1) and (2) below are calculated based on the shape of the dimples on the finished ball in which such treatment has been entirely completed.

Dimple Parameter (1)

The total number of dimples is set in a range of at least 250 but not more than 500. In this case, the lower limit may be set to preferably at least 280, more preferably at least 300, and even more preferably at least 310. The upper limit may be set to preferably not more than 450, more preferably not more than 420, even more preferably not more than 400, and most preferably not more than 350.

Dimple Parameter (2)

To improve aerodynamic performance, the dimple surface coverage (SR), defined as the sum of the surface areas on a hypothetical sphere that are circumscribed by the edges of the respective dimples as a proportion of the surface area of the hypothetical sphere, while not subject to any particular limitation, is set to preferably at least 70%. The SR may be set to more preferably at least 71%, and even more preferably at least 72%.

Dimple Parameter (3)

To improve the aerodynamic performance, the dimple volume ratio (VR), defined as the sum of the volumes of individual dimple spaces below a flat plane circumscribed by the edge of each dimple on a golf ball as a proportion of the volume of the hypothetical sphere were the golf ball to have no dimples on the surface, is set to from 1.00 to 1.40%. The lower limit is preferably at least 1.05%. The upper limit is preferably not more than 1.35%, more preferably not more than 1.30%, and even more preferably not more than 1.26%. In cases where the volume ratio is larger than the above range, the trajectory may become too low, as a result of which the ball may not travel far enough on shots taken at a low head speed. On the other hand, when the volume ratio is smaller than the above range, a sufficient distance-reducing effect may not be achieved on shots taken at a high head speed.

The shapes of the dimples are not limited to circular shapes, and may also be suitably selected from among, for example, polygonal, tear-shaped and oval shapes. Setting the number of dimple types to at least three, and preferably at least five, makes it possible for the dimples to cover a higher proportion of the spherical surface. Also, by interspersing large and small dimples, the surface coverage can be increased to the specified range. Because this enables extreme fluctuations in the coefficient of lift (CL) within the low-velocity region to be suppressed, the ball trajectory can be made relatively low, thus making it easier to elicit the advantageous effects of the invention.

The golf ball of the invention can be made to conform with the Rules of Golf for competitive play, and may be formed to a diameter of not less than 42.67 mm. It is suitable to set the weight to generally not less than 45.0 g, and preferably not less than 45.2 g, but not more than 45.93 g.

The golf ball of the invention is composed of the above-described core and the cover of at least one layer, and has a plurality of dimples on the surface of the outermost layer of the cover. The ball as a whole has a deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of at least 4.0 mm, preferably at least 4.1 mm, and more preferably at least 4.2 mm. The deflection has an upper limit which is not more than 6.0 mm, preferably not more than 5.8 mm, more preferably not more than 5.6 mm, even more preferably not more than 5.3 mm, and most preferably not more than 5.0 mm. If the deflection is too small, the distance traveled by the ball on shots taken a high head speed may be excessive, making it impossible to achieve the reduction in distance on high HS shots that is the object of the invention. On the other hand, if the deflection is too large, the ball may have a poor durability to cracking and may have an excessively soft feel on impact.

The initial velocity of the ball, as measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument is from 70.0 to 76.0 m/s. The lower limit value is preferably at least 72.0 m/s, more preferably at least 73.0 m/s, and even more preferably at least 74.0 m/s. The upper limit value is preferably not more than 75.8 m/s, and more preferably not more than 75.7 m/s.

In the golf ball of the invention, although not subject to any particular limitation, from the standpoint of suppressing the reduction in distance on shots taken at a low head speed while markedly reducing the distance on shots taken at a high head speed, it is preferable for the dimple volume ratio and the deflection described above to satisfy the relationship expressed by the formula:

4×VR+deflection=8.0 to 10.0.

When the relationship between the dimple volume ratio and the deflection does not satisfy the above formula, a sufficient distance-reducing effect may not be achieved on shots taken at a high head speed, in addition to which a good feel and a sufficient durability to cracking may not be attained. In the present invention, the above formula serves as an indicator representing a proper relationship between the dimples and the ball construction that ensures the distance traveled by the ball when struck at a low head speed while suppressing the distance traveled when struck at a high head speed.

As explained above, the golf ball of the invention greatly reduces the distance traveled by the ball on shots taken at a high head speed. In such cases, although not subject to any particular limitation, it is preferable for the ball, under the conditions of a head speed of 54 m/s, a ball initial velocity of 78.0±0.5 m/s, a launch angle of 9.7±0.5° and an initial backspin rate of 2,700±100 rpm, to have a total distance of not more than 290 yards.

Also, it is preferable for the difference in total distance traveled by the ball depending on the magnitude of the head speed to be small. Although not subject to any particular limitation, it is recommended that the ball have a ratio of total distance traveled when struck at a head speed of 54 m/s to total distance traveled when struck at a head speed of 35 m/s (HS54/HS35) of at least 1.30. The lower limit value in this ratio is preferably at least 1.35, more preferably at least 1.38, and even more preferably at least 1.40. The upper limit value is preferably not more than 1.50.

As described above, in this invention, it is possible to substantially reduce the distance traveled by the ball on high HS shots while at the same time minimizing the decrease in distance traveled by the ball on low HS shots. As a result, there can be obtained a superior golf ball for competitors having a low head speed.

EXAMPLES

The following Examples and Comparative Examples are provided by way of illustration and not by way of limitation.

Examples 1 to 4, Comparative Examples 1 to 5

The rubber compositions shown in Table 1 were prepared, then molded and vulcanized at 155° C. for 15 minutes to produce solid cores. Numbers in the table indicate parts by weight.

TABLE 1 A B C D E F Polybutadiene 75 74 85 70 100 100 rubber Isoprene rubber 25 13 15 30 0 0 Butyl rubber 0 13 0 0 0 0 Peroxide 1 0.6 0.6 0.6 0.6 0.6 0.6 Peroxide 2 0.6 0.6 0.6 0.6 0.6 0.6 Barium sulfate 22 0 24 20 21.7 25.8 Zinc oxide 4 4 4 4 4 4 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 Zinc diacrylate 19.5 19.5 15.0 24.0 20.0 11.0 Zinc distearate 5 5 5 5 5 5 Calcium 0 19.8 0 0 0 0 carbonate Zinc salt 0 0 0 0 0.1 0 of penta- chlorothio- phenol Specific gravity 1.162 1.162 1.162 1.162 1.162 1.162

Trade names of the materials in the table are as follows.

-   Polybutadiene rubber: Available under the trade name “BR 01” from     JSR Corporation. -   Isoprene rubber: Available under the trade name “IR 2200” from JSR     Corporation. -   Butyl rubber: Available under the trade name “Bromobutyl 2222” from     Japan Butyl Co., Ltd. -   Peroxide 1: Dicumyl peroxide, available under the trade name     “Percumyl D” from NOF Corporation. -   Peroxide 2: 1,1-Bis(t-butylperoxy)cyclohexane, available under the     trade name “Perhexa C-40” from NOF Corporation. -   Barium sulfate: Available under the trade name “Precipitated Barium     Sulfate 100” from Sakai Chemical Industry Co., Ltd. -   Zinc oxide: Available from Sakai Chemical Industry Co., Ltd. -   Antioxidant: Available under the trade name “Nocrac NS-6” from Ouchi     Shinko Chemical Industry Co., Ltd. -   Zinc diacrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. -   Zinc distearate: Available under the trade name “Zinc Stearate G”     from NOF Corporation.

Next, the cover material shown in Table 2 below was injection-molded over the core, thereby obtaining a golf ball in which the core is encased within an inner cover layer and an outer cover layer of given thicknesses.

TABLE 2 Cover material (pbw) (1) (2) HPF 2000 100 0 Pandex T8290 0 25 Pandex T8295 0 75 Polyisocyanate compound 0 9 Thermoplastic elastomer 0 15 Titanium oxide 0 3.5 Polyethylene wax 0 1.5

Trade names of the materials in the table are as follows.

-   HPF 2000: An ionomeric resin available from E.I. DuPont de Nemours &     Co. -   Pandex: MDI-PTMG type thermoplastic polyurethanes available from DIC     Bayer Polymer. -   Polyisocyanate compound: 4,4′-Diphenylmethane diisocyanate. -   Thermoplastic elastomer: Available under the trade name “Hytrel     4001” from DuPont-Toray Co., Ltd. -   Titanium oxide: Available under the trade name “Tipaque R550” from     Ishihara Sangyo Kaisha, Ltd. -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd.

Simultaneous with injection molding of the cover, numerous dimples were formed on the surface of the cover, after which the cover was spray-painted. In each example and comparative example, the dimples on the surface of the ball after painting satisfied the parameters shown in Tables 3 to 6 below. In these tables, the dimple types designated as Da refer to dimples having a diameter of 3.7 mm or more, and the dimple types designated as Db refer to dimples having a diameter of less than 3.7 mm.

Here, referring to FIG. 2, the dimple depth (DP) in the tables is the vertical distance from a hypothetical flat plane L, traced by connecting the positions where the dimple meets land areas, to the bottom (deepest position) of the dimple. The dimple diameter (DM), as shown in FIG. 2, is the diameter (span) between positions where the dimple portion is tangent with land areas (non-dimple-forming portions), i.e., between the high points of the dimple portion. In most cases, the golf ball has been subjected to painting or the like. In such balls, the dimple diameter and depth refer to the diameter and depth after the coat of paint has been applied.

With regard to the dimple patterns in the tables, the dimple patterns for Examples 1 to 3 and for Comparative Examples 3 to 5 are shown in Table 3 (FIG. 3), the pattern for Example 4 is shown in Table 4 (FIG. 4), the pattern for Comparative Example 1 is shown in Table 5 (FIG. 3), and the pattern for Comparative Example 2 is shown in Table 6 (FIG. 5). These diagrams are all top views of the ball. In the respective examples, the bottom views of the ball have the same pattern as the top views, and are thus omitted.

TABLE 3 Examples 1 to 3, Number Comparative Examples 3 to 5 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 24 4.5 0.16 1.34 Da-II 150 4.3 0.16 1.22 Da-III 66 3.7 0.15 0.86 Db-I 18 2.7 0.12 0.36 Db-II 6 2.5 0.12 0.31 Da-IV 48 4.3 0.17 1.30 Da-V 12 3.8 0.16 0.96 Db-III 6 3.4 0.16 0.75 Db-IV 6 3.3 0.15 0.66

TABLE 4 Number Example 4 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 12 4.6 0.23 1.97 Da-II 174 4.4 0.22 1.78 Da-III 18 4.0 0.21 1.40 Da-IV 6 3.8 0.20 1.21 Db-I 6 3.3 0.18 0.81 Db-II 36 2.4 0.13 0.32 Da-V 54 4.5 0.27 2.25 Db-III 6 3.4 0.16 0.75

TABLE 5 Number Comparative Example 1 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 24 4.5 0.16 1.39 Da-II 150 4.3 0.16 1.26 Da-III 66 3.7 0.16 0.89 Db-I 18 2.7 0.12 0.37 Db-II 6 2.5 0.12 0.31 Da-IV 48 4.3 0.18 1.35 Da-V 12 3.8 0.17 0.99 Db-III 6 3.4 0.16 0.75 Db-IV 6 3.3 0.15 0.66

TABLE 6 Number Comparative Example 2 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 12 4.6 0.16 1.28 Da-II 222 4.4 0.16 1.16 Da-III 36 3.8 0.15 0.80 Db-I 12 2.6 0.12 0.58 Da-IV 12 4.4 0.17 0.25 Da-V 24 3.8 0.16 1.25 Db-II 6 3.5 0.16 0.86 Db-III 6 3.4 0.15 0.7

Various properties of the resulting golf balls were investigated by the following methods. The results are shown in Tables 7 and 8.

Deflection of Solid Core and Finished Ball

The solid core and the finished ball were placed on a hard plate, and the deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

Cover Material Hardness (Shore D Hardness)

The cover-forming material was formed under applied pressure to a thickness of about 2 mm and the resulting sheet was held at 23° C. for 2 weeks, following which the Shore D hardness of the sheet was measured in accordance with ASTM D2240.

Initial Velocity

The initial velocity of the ball was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The ball was held isothermally in a 23±1° C. environment for at least 3 hours, then tested in a chamber at a room temperature of 23±2° C. The ball was hit using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s). One dozen balls were each hit four times. The time taken for the ball to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity (m/s) of the ball. This cycle was carried out over a period of about 15 minutes.

Flight Performance

A driver manufactured by Bridgestone Sports Co., Ltd. (TOURSTAGE X-DRIVE 701 (2009 model; loft angle, 9.5°)) was mounted on a swing robot, and the distance traveled by the ball when hit at a head speed (HS) of 54 m/s or 35 m/s was measured. The initial conditions here were set using Bridgestone Golf e5 balls (manufactured by Bridgestone Sports Co., Ltd.), which have been sold in the United States since November 2009. When this ball was hit at a head speed of 54 m/s, the initial velocity was set to 78.0±0.5 m/s, the launch angle was set to 9.7±0.5°, and the initial backspin rate was set to 2,700±100 rpm. When the ball was hit at a head speed of 35 m/s, the initial velocity was set to 53.0±0.5 m/s, the launch angle was set to 14.0±0.5°, and the initial backspin rate was set to 2,700±100 rpm.

Durability to Cracking

The durability of the golf ball was evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). This tester functions so as to fire a golf ball pneumatically and cause it to repeatedly strike two metal plates arranged in parallel. The incident velocity against the metal plates was set to 43 m/s. For each type of ball, the durability of the ball was judged to be the number of shots that had been taken with the ball when the velocity ratio (rebound velocity/incident velocity) of the ball fell below 97% of the average value for the first ten shots, and was rated according to the following criteria.

Good: 50 or more shots

NG: less than 50 shots

TABLE 7 Example 1 2 3 4 Core Formulation A B C A Diameter (mm) 37.3 37.3 37.3 37.3 Deflection (mm) 5.0 5.0 6.8 5.0 Inner Material (1) (1) (1) (1) cover Material hardness (Shore D) 48 48 48 48 layer Thickness (mm) 1.5 1.5 1.5 1.5 Outer Material (2) (2) (2) (2) cover Material hardness (Shore D) 55 55 55 55 layer Thickness (mm) 1.2 1.2 1.2 1.2 Dimples Number of dimple types 9 types 9 types 9 types 8 types Number of dimples 336 336 336 312 SR value (%) 76.0 76.0 76.0 74.4 VR value (%) 1.06 1.06 1.06 1.25 Ball Diameter (mm) 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 Deflection (mm) 4.3 4.3 5.6 4.3 Initial velocity (m/s) 75.6 73.1 75.6 75.6 Formula: 4 × VR + deflection 8.5 8.5 9.8 9.3 Flight HS Carry (y) 262.5 255.5 254.5 256.5 54 m/s Total distance (y) 286.5 278.5 282.2 285.4 HS Carry (y) 166.5 160.5 162.3 165.8 35 m/s Total distance (y) 192.5 188.0 188.8 192.0 Difference in total distance (y) 94.0 90.5 93.4 93.4 Total distance ratio HS54/HS35 1.49 1.48 1.49 1.49 Durability to cracking good good good good

TABLE 8 Comparative Example 1 2 3 4 5 Core Formulation A A D E F Diameter (mm) 37.3 37.3 37.3 37.3 37.3 Deflection (mm) 5.0 5.0 3.8 5.0 9.0 Inner Material (1) (1) (1) (1) (1) cover Material hardness (Shore D) 48 48 48 48 48 layer Thickness (mm) 1.5 1.5 1.5 1.5 1.5 Outer Material 2 2 2 2 2 cover Material hardness (Shore D) 55 55 55 55 55 layer Thickness (mm) 1.2 1.2 1.2 1.2 1.2 Dimples Number of dimple types 9 types 8 types 9 types 9 types 9 types Number of dimples 336 330 336 336 336 SR value (%) 75.6 78.0 76.0 76.0 76.0 VR value (%) 0.92 0.88 1.06 1.06 1.06 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 45.3 Deflection (mm) 4.3 4.3 3.3 4.3 7.4 Initial velocity (m/s) 75.6 75.6 75.6 77.3 75.8 Formula: 4 × VR + deflection 8.0 7.8 7.5 8.5 11.6 Flight HS Carry (y) 274.9 275.5 269.0 268.0 243.0 54 m/s Total distance (y) 294.8 295.9 292.2 293.0 274.5 HS Carry (y) 169.0 169.1 168.0 172.0 156.0 35 m/s Total distance (y) 191.6 191.0 192.6 194.6 184.2 Difference in total distance (y) 103.2 104.9 99.6 98.4 90.3 Total distance ratio HS54/HS35 1.54 1.55 1.52 1.51 1.49 Durability to cracking good good good good NG

From the results in Tables 7 and 8, it was confirmed that, compared with the golf balls in Comparative Examples 1 to 5, the golf balls in Examples 1 to 4 of the present invention had a decrease in distance on shots taken at low head speed that was suppressed relative to the large reduction in distance on shots taken at a high head speed. That is, the working examples of the invention were confirmed to be golf balls which had a small difference between the distance at high head speed and the distance at low head speed, and were thus able to achieve a superior distance in the low HS range while suppressing the distance in the high HS range. The results obtained for the golf balls in Comparative Examples 1 to 5 were as follows.

In Comparative Example 1, because the dimple volume ratio was small, the total distance on shots taken at a head speed of 54 m/s exceeded 290 yards, and there was a large difference in total distance between when the ball was struck at a head speed of 54 m/s and when it was struck at a head speed of 35 m/s, the total distance ratio HS54/HS35 was large.

In Comparative Example 2, because the dimple volume ratio was small, the total distance on shots taken at a head speed of 54 m/s exceeded 290 yards, and there was a large difference in total distance between when the ball was struck at a head speed of 54 m/s and when it was struck at a head speed of 35 m/s, the total distance ratio HS54/HS35 was large.

In Comparative Example 3, because the ball had a small deflection, the total distance traveled by the ball when struck at a head speed of 54 m/s exceeded 290 yards.

In Comparative Example 4, because the initial velocity was high, the total distance of the ball on shots taken at a head speed of 54 m/s exceeded 290 yards.

In Comparative Example 5, because the ball had a large deflection, the durability to cracking was poor. 

1. A golf ball comprising a solid core, a cover of at least one layer, and a plurality of dimples on a surface of an outermost layer of the cover, wherein the dimples number at least 250 but not more than 500 and have a volume ratio (VR) of from 1.00 to 1.40%, and the ball has a deflection, when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), of from 4.0 to 6.0 mm and an initial velocity, as measured using an initial velocity measuring apparatus of the same type as a USGA drum rotation-type initial velocity instrument, of from 70.0 to 76.0 m/s.
 2. The golf ball of claim 1, wherein the ball, upon the conditions of a head speed of 54 m/s, a ball initial velocity of 78.0±0.5 m/s, a launch angle of 9.7±0.5° and an initial backspin rate of 2,700±100 rpm, has a total distance of not more than 290 yards.
 3. The golf ball of claim 1, wherein the ball has a ratio of total distance traveled when struck at a head speed of 54 m/s to total distance traveled when struck at a head speed of 35 m/s (HS54/HS35) of from 1.30 to 1.50.
 4. The golf ball of claim 1, wherein the dimple volume ratio and the deflection satisfy the relationship expressed by the formula: 4×VR+deflection=8.0 to 10.0. 